8025491: Javac regression test tools/javac/T8003967/DetectMutableStaticFields.java failing

Summary: Making HtmlTree.NONENCODING_CHARS final
Reviewed-by: jjg

src/share/classes/com/sun/tools/doclets/formats/html/markup/HtmlTree.java

@@ -167,7 +167,7 @@ public class HtmlTree extends Content {
     /**
      * A set of ASCII URI characters to be left unencoded.
      */
-    public static BitSet NONENCODING_CHARS = new BitSet(256);
+    public static final BitSet NONENCODING_CHARS = new BitSet(256);
 
     static {
         // alphabetic characters